The applications of graphene and transition metal dichalcogenides in electronics are limited by their zero-bandgap and low mobility, respectively. Here, the authors demonstrate the potential of an emerging layered material—black phosphorous—for thin film electronics and infrared optoelectronics.

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Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics

This is the post-peer reviewed version of the following article: A. Castellanos-Gomez et al. “Isolation and characterization of few-layer black phosphorus”. 2D Matererials, 2014, 1(2) 025001 doi:10.1088/2053-1583/1/2/025001 Which has been published in final form at: http://iopscience.iop.org/2053-1583/1/2/025001

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Isolation and characterization of few-layer black phosphorus

We mechanically exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact number of layers is unambiguously determined by atomic force microscopy and high-resolution Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.

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Photoluminescence emission and Raman response of monolayer MoS 2 , MoSe 2 , and WSe 2

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe2, which shows stark contrast from the well-explored MoS2 where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe2 is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe2, identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.

Thermally driven crossover from indirect toward direct bandgap in 2D Semiconductors: MoSe2 versus MoS2

Sodium-ion batteries have recently attracted significant attention as an alternative to lithium-ion batteries because sodium sources do not present the geopolitical issues that lithium sources might. Although recent reports on cathode materials for sodium-ion batteries have demonstrated performances comparable to their lithium-ion counterparts, the major scientific challenge for a competitive sodium-ion battery technology is to develop viable anode materials. Here we show that a hybrid material made out of a few phosphorene layers sandwiched between graphene layers shows a specific capacity of 2,440 mA h g−1 (calculated using the mass of phosphorus only) at a current density of 0.05 A g−1 and an 83% capacity retention after 100 cycles while operating between 0 and 1.5 V. Using in situ transmission electron microscopy and ex situ X-ray diffraction techniques, we explain the large capacity of our anode through a dual mechanism of intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy. The presence of graphene layers in the hybrid material works as a mechanical backbone and an electrical highway, ensuring that a suitable elastic buffer space accommodates the anisotropic expansion of phosphorene layers along the y and z axial directions for stable cycling operation. The sodiation–desodiation properties of few-layer phosphorene are mostly preserved by sandwiching the material between graphene layers, a behaviour that makes phosphorene–graphene hybrids a potentially suitable anode material for sodium-ion batteries.

A phosphorene–graphene hybrid material as a high-capacity anode for sodium-ion batteries

A stochastic random network model is proposed for the structure of ${\mathrm{Ge}}_{\mathrm{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$, ${\mathrm{Ge}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$, ${\mathrm{Si}}_{\mathrm{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$, and ${\mathrm{Si}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ (x\ensuremath{\le}0.33) glasses. This model is constructed to explain the existence of two types of microcrystalline states induced by photoirradiation above or below the threshold intensity. This model characterizes the glass structure by one parameter P which is related to the existing probability of the edge-sharing bonds between the tetrahedral ${\mathrm{MX}}_{4}$ molecules relative to the corner-sharing bonds. P depends only on the species of atoms forming the glass and not on x. In order to prove the validity of the present model, Raman scattering experiments were made and the x dependence of the intensity ratio of the ${A}_{1}^{c}$ companion peak to the ${A}_{1}$ peak, I(${A}_{1}^{c}$)/I(${A}_{1}$), was obtained. From the viewpoint of phonon localization, the ${A}_{1}$ mode is assigned to the breathing mode of ${\mathrm{MX}}_{4}$ molecules and the ${A}_{1}^{c}$ mode to the vibration of chalcogen atoms on the edge-sharing double bonds. The x dependence of the intensity ratio I(${A}_{1}^{c}$)/I(${A}_{1}$) calculated by the present model is in good agreement with the experimentally obtained ratio. The P obtained increases in order from ${\mathrm{Ge}}_{\mathrm{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$, ${\mathrm{Ge}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$, ${\mathrm{Si}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ to ${\mathrm{Si}}_{\mathrm{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ with the same order of tendency of getting edge-sharing bonds in the crystals. The value of P is independent of the method of making the amorphous but it can be changed by photoirradiation. P decreases with irradiation below the threshold intensity, but it increases with irradiation above the threshold. The local energy in the glass is lower in the corner-sharing bonds, but the total energy is lowest in the same structure as the crystal. The threshold irradiation intensity for Se glass is less than one-hundredth of that for ${\mathrm{GeSe}}_{2}$ glass.

Stochastic random network model in Ge and Si chalcogenide glasses.

Raman and infrared reflection spectroscopy in black phosphorus

Lattice vibrations were investigated by Raman spectroscopy in the layered materials $2H$-Mo${\mathrm{S}}_{2}$ up to 180 kbar, $2H$-Mo${\mathrm{Se}}_{2}$ up to 180 kbar, and $2H$-Mo${\mathrm{Te}}_{2}$ up to 80 kbar. One ${A}_{1g}$ and two ${E}_{2g}$ modes were observed. The energies of the rigid-layer modes rapidly increase with pressure, but the increase becomes slow above 50 kbar. The interlayer and intralayer shear force constants are obtained as a function of pressure with the use of the linear-chain model.

High-pressure Raman spectroscopy in the layered materials 2 H -Mo S 2 , 2 H -Mo Se 2 , and 2 H -Mo Te 2

The semiconducting ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (0.35lx\ensuremath{\le}1) is investigated in detail by optical measurements. The optical spectrum shows that ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ has a clear optical gap over the whole semiconducting compositional range. It is clearly shown by resonant Raman scattering measurements that the origin of this gap is a charge-density wave (CDW) accompanied by the breathing-mode distortion. No apparent effect of Pb substitution can be observed in the reflectivity spectra for the sample with xg0.7. For x\ensuremath{\le}0.7, the CDW gap decreases with x and simultaneously the absorption tail extends into the gap. This spectral behavior can be well interpreted in terms of the recently proposed theoretical model with emphasis on a local CDW and/or a substantial energy difference between Pb and Bi sites arising from the strong electron-phonon interaction. Thus it can be concluded that the strong electron-phonon interaction plays an essential role in the semiconducting phase of this material as well as in the metallic phase.

Electronic states of BaPb1-xBixO3 in the semiconducting phase investigated by optical measurements.

A combination of x-ray-diffraction measurements, using a diamond-anvil cell, and electrical-resistance measurements, employing an octahedral-anvil press, has been used to elucidate the behavior of GeSe and GeTe at high pressure. In GeSe, x-ray data show that the array of six-membered rings in chair conformation along the $a$ direction is the most compressible. The electrical resistivity decreases with increasing pressure and exhibits an order of magnitude drop at about 25 GPa. GeTe transforms from the rhombohedral to the NaCl-type structure at 3 GPa and, at 18 GPa, transforms into a possibly orthorhombic structure with a space group $Pbcn,$ accompanied by a remarkable rise in electrical resistivity. General trends in the pressure-induced structural phase transitions of group-IV and group-VI compounds are discussed in terms of ionicity and covalency.

Shunji Sugai

Papers

Stochastic random network model in Ge and Si chalcogenide glasses.

Raman and infrared reflection spectroscopy in black phosphorus

High-pressure Raman spectroscopy in the layered materials 2 H -Mo S 2 , 2 H -Mo Se 2 , and 2 H -Mo Te 2

Electronic states of BaPb1-xBixO3 in the semiconducting phase investigated by optical measurements.

Structural and electrical properties of GeSe and GeTe at high pressure